1. Field of the Invention
The present invention is broadly concerned with improved fluidized bed precipitators particularly designed for use in treating phosphorus-containing collected animal waste liquids from concentrated animal feeding operations. The precipitators have an upright primary tubular section and an upper settling section oriented at an oblique angle relative to the upright primary tubular section in order to facilitate solids precipitation. In other aspects of the invention, fluidized bed precipitators are provided having pressure sensor-controlled systems for monitoring the buildup of solids and to periodically withdraw collected solids from the precipitators without clogging of outlet ports or the like.
2. Description of the Prior Art
The handling of livestock manures or wastes is a significant problem in Concentrated Animal Feeding Operations (CAFOs) such as cattle feedlots. These wastes contain nutrients such as carbon, nitrogen, and phosphorus, and is often flushed with water from animal confinement areas into a lagoon for treatment and storage. In addition, to avoid the over-filling of the lagoon that would otherwise result from the net inputs of waste and rainwater, an effluent stream is withdrawn and directed as irrigation water onto soil supporting crops.
In the lagoon, anaerobic microbes remove organic carbon compounds by converting them into the volatile gases carbon dioxide and methane. These gases may escape to the atmosphere or be collected as biogas and combusted for energy recovery.
Nitrogen, which exists primarily in forms of ammonia in the lagoon, may partly volatilize into the atmosphere from the lagoon or during irrigation. The crop acreage is typically calculated to allow for uptake by the crops of the applied nitrogen from the soil, thus minimizing movement of nitrogen in ground and surface water beyond the farm's boundaries. In addition, to reduce the amount of ammonia escaping to the atmosphere and/or to reduce the demand for nitrogen uptake by crops, processes are under development for removal of nitrogen by microbes. In these processes, aerobic microbes convert ammoniacal forms of nitrogen to oxidized forms such as nitrate, followed by conversion of the oxidized forms to nitrogen gas by anaerobic microbes. The nitrogen gas escapes to the atmosphere, thus removing nitrogen.
Unlike carbon and nitrogen, phosphorus cannot volatilize from the system. In the lagoon, it exists partly as inorganic phosphorus, organic phosphorus and as orthophosphate phosphorus (OP), none of which can evaporate or be converted by microbes to gaseous forms. Some phosphorus may be removed from the wastewater by settling of phosphorus-containing solids to the lagoon bottom; however, this process does not ultimately remove the nutrient from the system, and appreciable phosphorus remains dissolved in the lagoon water. The irrigated crops typically take up from the soil less phosphorus than that applied in the irrigation water, because the acreage has been calculated for nitrogen removal, which requires less acreage. The soil therefore absorbs and accumulates phosphorus which can be carried by surface waters beyond farm boundaries, risking depletion of oxygen in those waters by accelerating growth of oxygen-consuming aquatic organisms. New processes mentioned above for nitrogen removal will, if anything, worsen the phosphorus excess as the acreage requirements for nitrogen removal shrink. Measures to reduce the phosphorus content of the lagoon effluent must be therefore be considered.
Methods to remove solids, such as centrifugation, filtration, and settling, will remove much of the insoluble phosphorus. For removal of soluble phosphorus, three methods may be considered: (1) removal of phosphate-accumulating microbes; (2) precipitation by iron or calcium addition; and (3) precipitation as struvite (magnesium ammonium phosphate, Mg NH4 PO4.6H2O, (MAP)). The latter method is often preferred, inasmuch as the struvite can be grown to large, easily separable particles. Precipitation of struvite as a phosphorus recovery method has been investigated since at least 1969, as exemplified by a report to the US Department of Interior entitled Ultimate Disposal of Phosphate from Waste Water by Recovery as Fertilizer-Phase I-Final Report, suggesting the use of various additives to force the precipitation of struvite.
Struvite precipitation has been suggested as a process for removing phosphorus from lagoon wastewater. In this process, the concentration of magnesium (Mg2+) ions, ammonium NH4+), and phosphate (PO43−) ions must be brought high enough that the equilibrium solubility product of struvite is exceeded. In addition, there must be enough Mg and ammonium (NH4+) ions present in stoichiometric comparison to the phosphorus that, as precipitation occurs, the solubility product will continue to be exceeded until the phosphorus reduction goal has been met. Although lagoon wastewater usually contains ammonium and some magnesium, magnesium is often added in excess of the stoichiometric ratio, in order to drive the precipitation reaction to remove the targeted amount of phosphorus. In addition, pH elevation by chemical addition may be necessary to achieve a thermodynamic state of low struvite solubility. The main advantage of struvite precipitation is that the precipitate can be made to form a coarse-grained material that is easily drained of its water and is thus less expensive to handle and transport.
A fluidized bed is a common, efficient, and flexible piece of equipment used in many chemical production processes, including precipitation, crystal (or particle) growth, catalyzed reaction, bio-reaction, polymerization, particle coating, mass transfer, heat exchange, and solid drying processes. Fluidized beds have been used for precipitating, growing, and retaining solid particles since at least 1970 (see, U.S. Pat. No. 3,510,266). In a fluidized bed, solid particles flow much in the manner of a fluid as they are suspended and moved by an upward flowing fluid, maximizing solids-fluid contact. By comparison, in a packed bed, fluid often develops channels through the solids so that much of the solid surface area is not used for its intended purpose. The advantages of a fluidized bed include (1) the ability to operate as a continuous flow process (vs. batch), (2) good mixing of both mass and energy without the use of stirrers or other mixing equipment, (3) good liquid-solids and/or liquid-solids-gas contact, and (4) solids/liquid separation that takes advantage of the forces of gravity and upward flow at flow rates fine-tuned to cause separation of two materials. The solid particles in a fluidized bed can serve as reactant, catalyst, product, or seed material for precipitation and particle growth. The solid particles are fluidized by the upward flow of a fluid, gas and/or liquid, which provides for good contact between the surface of the solid particles and the upward flowing fluid. In addition to an upward flowing fluid, a fluidized bed might also have either (1) another upward flowing fluid, or (2) another downward flowing fluid of higher specific gravity than the upward flowing fluids.
The solid particles are fluidized by fine-tuning the overall upward velocity so that the average sum of forces on the particles (i.e., frictional between particles and fluid, gravitational) in the bed is close to zero, thus maintaining the density of particles in the bed between an upper and lower limit. Because the velocities of upward flowing fluids are greater in the center of a column and weaker toward the walls of the column where drag forces are higher, the solid particles in the center of a fluidized bed will be generally rising while solids near the wall will be generally falling, while mostly, if optimized being retained in the bed.
In fluidized bed systems, some undesired entrainment occurs as solids are carried out of the fluidized bed with the exiting upward-flowing fluid. Fluidized beds of relatively narrow particle-size distribution exhibit two distinct zones, a lower, denser zone and an upper, sparser zone. The lower zone is the main fluidized region, where reactions and interactions occur. The distinct line separating these two zones is called the “freeboard height,” and the upper zone, between the main bed and the fluid exit, is called the “freeboard.” Some solids from the main fluidized bed are propelled into the freeboard, and they either fall back into the fluidized bed or are carried out with the exiting liquid. The higher the freeboard height, the fewer solid particles will escape. However, for any given flow velocity, particles smaller than a certain size will escape the column, regardless of how high the freeboard height is raised. The height above which entrainment doesn't decrease much with added freeboard height is called the transport disengaging height (TDH). Additionally, when turbulence is present in the fluidized bed (this can occur for flow rates with Reynold's numbers above 2,320), medium and larger size particles can also escape. Therefore, flow rates are limited to below which turbulence causes an unacceptable amount of solids to escape the column.
For the best operation of a fluidized bed, particle size should be narrow, so that flow can be set at a precise rate for good fluidization with minimal loss of particles by entrainment. Optimized flow is such that the flow velocity in the column falls between the minimum flow velocity for fluidization (umf) and the terminal velocity (ut) for the greatest number of particles in a size range, calculated as follows:
                              u          t                =                                            4              ⁢                              d                p                            *                              (                                                      ρ                    s                                    -                                      ρ                    g                                                  )                            ⁢              g                                      3              ⁢                              ρ                g                            ⁢                              C                D                                                                        (        1        )            
where dp=particle diameter                ρs=particle density        ρg=fluid density        CD=particle drag coefficient, which can be determined experimentally or via available equations, such as the Haider and Levenspiel equation.        
Terminal velocity is equal to the maximum upward flow rate against which the force of gravity will cause a given particle to resist entrainment. By the equation shown, it is obvious that particles of different sizes, densities, and/or surface characteristics have different maximum flow rates above which they will be entrained in the same fluid.
Likewise, the minimum upward velocity for which a bed of particles separate and become fluidized is determined by particle size, density, and surface characteristics, as shown by the following quadratic equation that can be solved by iterative analysis:
                                                        1.75                                                ɛ                  mf                  3                                ⁢                                  Φ                  s                                                      *                                                                                d                    p                                    ⁢                                      u                    mf                                    ⁢                                      ρ                    g                                                  μ                                              +                                                    150                ⁢                                  (                                      1                    -                                          ɛ                      mf                                                        )                                                                              ɛ                  mf                  3                                ⁢                                  Φ                  s                  2                                                      *                                                            d                  p                                ⁢                                  u                  mf                                ⁢                                  ρ                  g                                            μ                                      =                                            d              p              3                        ⁢                                          ρ                g                            ⁡                              (                                                      ρ                    s                                    -                                      ρ                    g                                                  )                                      ⁢            g                                μ            2                                              (        2        )            
where umf=minimum velocity for fluidization                dp=particle diameter        ρs=particle density        ρg=fluid density        εnif=voidage in bed at minimum fluidizing conditions        Fs=sphericity (surface area of sphere/surface area of particle) of same volume        μ=actual fluid viscosity        
The relationships among minimum fluidization velocity and particle characteristics are more easily shown in the simplified equation for small particles, where umf can be solved for analytically:
            u      mf        =                                                      d              p              2                        ⁡                          (                                                ρ                  s                                -                                  ρ                  g                                            )                                *          g                          150          ⁢          μ                    *                                    ɛ            mf            3                    ⁢                      Φ            s            2                                    1          -                      ɛ            mf                                ,            for      ⁢                          ⁢              Re                  p          ,          m                      <    20    ,            where      ⁢                          ⁢              Re                  p          ,          m                      =                            d          p                ⁢                  u          mf                ⁢                  ρ          g                    μ      
KUNII, D. and Octave Levenspiel. Fluidization Engineering. 2nd Ed. Butterworth-Heinemann, 1991. p. 80.
These equations demonstrate that particles that are denser, larger, and more spherical have higher terminal velocities and higher minimum fluidization velocities, while particles that are less dense, smaller, and less spherical have lower terminal velocities and lower minimum fluidization velocities. Therefore, a broad particle size distribution creates problems, since at any given flow rate, more particles will either not be fluidized or will escape the column than when particle size distribution is narrow. Accordingly, costly measures are often taken to create or purchase solids that maintain a uniform size. The problem is exacerbated in precipitation and/or crystal growth applications that inherently require broad particle distribution, since particles grow from small to large, chip away from larger particles, then grow again.
For precipitation applications, a fluidized bed is typically a vertical column, of constant diameter, into which relatively ion-rich liquid enters at the bottom and relatively ion-poor liquid exits at the top of the column. The precipitated solids form and grow in the fluidized bed until they are large enough to be harvested. Often, additives are injected into the column to adjust pH or to enhance the level of a component ion of the desired precipitate. Particles form by at least three mechanisms: nucleation (ions collide to form a small particle), agglomeration (smaller particles collide to form larger particles) and crystal growth (ions collide on a solid particle and are added to the mass of the particle). Crystal growth and agglomeration are mechanisms that form relatively stable crystals, whereas nucleation forms crystals that are more likely to disassociate, so that it is important to have a large amount of crystal surface area on “seed particles” in the column to form stable particles. A given mass of smaller particles supplies more surface area than the same mass of larger particles, so that smaller particles are more effective as precipitation sites. Particles are allowed to grow in the column, and periodically, larger particles are removed from the bottom of the column through a valve or plugged outlet.
The flow rate of a fluidized bed must be optimized, high enough to allow for good fluidization motion of the solid particles but low enough so that smaller precipitated particles are not carried out in the fluid flow at the top of the column, which is especially difficult in precipitation reactions where particle size distribution is broad. To accomplish balance, the top section of the column is sometimes of a larger diameter (giving lower velocity at a given volumetric flow rate) and the bottom section of a smaller diameter (giving higher velocity and better mixing at a given volumetric flow rate) to allow for good fluidization in the bottom section and good settling in the top section. Sometimes the fluidized bed comprises two cylindrical pieces, one of a smaller diameter and one of a larger diameter (the larger diameter section has been called an “expansion tank,” with a transition piece in between, while other times it is shaped like a cone, with the downward tip of the cone removed. In some processes, two concentric cylinders are used, so that upward flow, mixing and precipitation occur in the inner cylinder, while settling occurs in the outer cylinder.
One problem with existing fluidized bed precipitators is that small particles are difficult to retain in the column, even when the column is equipped with a large-diameter top section for improved settling. As is apparent by the Reynold's Number equation, which predicts turbulent flow (higher Re value indicates more likelihood of turbulence), larger column or pipe diameters create more turbulence at constant velocity than smaller diameters:
  Re  =            ρ      ⁢                          ⁢      VD        v  
where, μ=fluid density                V=free-stream fluid velocity        D=pipe or column diameter        v=fluid viscosityFor the same volumetric flow rate, a larger diameter yields a lower velocity, but the advantageous laminarizing effects of lowered velocity are partially offset by the turbulence-inducing effects of larger diameter. Additionally, turbulence can be exacerbated when gases, such as air or ammonia, are injected into the column as additives, since the gas bubbles cause turbulence and so can keep particles suspended in intended settling zones.        
A second problem with fluidized bed precipitators is that large particles are sometimes difficult to remove from the bed while the unit is operating. When a drain valve is opened, the bed can compress, causing clogging. Also, when a drain valve is opened, large amounts of water are released to withdraw a relatively small amount of solids.
A problem with fluidized beds used as struvite precipitators is that they can require one or more liquid and/or additives for operation, which require additive pumps and stirrers that complicate the system. Fluidized beds for precipitating phosphate ions have used liquid additives for adjusting pH and for contributing ammonia and/or magnesium ions to achieve supersaturation of component ions that favors the formation of struvite as a precipitated compound. Injecting liquids, however, requires that the system be equipped with additive pumps, additive tanks, and stirrers, when the liquid is a slurry. To form struvite, a magnesium source is often added (i.e., MgCl, MgSO4, MgO, Mg(OH)2), but the magnesium source is expensive and/or difficult to use. MgSO4, for example, is easy to use but expensive. On the other hand, MgO and Mg(OH)2, are less expensive, but because they have a very low solubility, usage requires pumping, stirring, and the addition of acid to increase their solubilities, all of which add expense.
A problem with fluidized bed precipitators for use on some cattle feedlots is that there are typically not two lagoons (wastewater ponds) in series through which all the feedlot water flows. Instead, rainfalls wash into a number of lagoons, which are quickly drained at high pumping rates to one or more final lagoons that are sourced for irrigation. To operate a precipitator as they are normally operated, between two lagoons in series, several very large systems capable of handling in the range of 300 to 1000 gpm could be needed on the feedlot, and these systems would only operate for short periods, a few days to a few weeks, at a time.
The following references describe various phosphorus removal processes including fluidized bed precipitator systems: U.S. Pat. Nos. 7,005,072; 6,994,782; 6,692,642; 6,846,343; 6,682,578; 6,776,816; 6,409,788; 5,993,503; 5,720,882; 5,443,613; 5,294,348; 4,576,627; 4,457,773; 4,431,543; 4,389,317; 4,321,078; 3,933,577; 3,892,539; 3,510,266; 3,476,510; 3,459,530; 3,348,910; 3,050,383; and 3,966,450.
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